In addition to their traditional propulsion functions, gas turbine engines are used as auxiliary power units, (APUs), aboard many types of aircraft, ground vehicles, and stationary installations to provide continuous shaft and/or pneumatic power. The shaft power is used to drive electric generators, hydraulic pumps, or other equipment requiring constant speed operation. The pneumatic power is used for main engine starting, cabin air-conditioning and pressurization, de-icing, air turbine motors, or other components requiring compressed air. When used aboard an aircraft, for example, the APU is typically mounted in a compartment located within the tail cone of the aircraft.
Historically, APU's have only been operated when the aircraft was on the ground. However, recent developments in aircraft design have witnessed the advent of twin engine aircraft capable of long distant, transoceanic flights. Examples of such aircraft are the Boeing 757, 767 and 777, currently under development, as well as the Airbus A300, A310, and A320. A disadvantage to the twin engine design is that when a main engine experiences an inflight shutdown the enormous burden of supplying the aircraft with power falls on the sole, remaining engine. Early on in the development of these aircraft, it was recognized that they would need an additional source of power while inflight. To meet this need it was proposed to start and operate the APU inflight at high altitudes.
During the operation of the APU, heat is rejected into the compartment from numerous sources including the engine skin, exhaust gases, and tailpipe, as well as the engine oil cooler, generator, and other compartment accessories. To prevent the temperature in the compartment from reaching unacceptable levels, a ventilating or cooling airflow must be provided through the compartment.
To remove this heat )an axial, vane type fan, driven off the APU gearbox, is usually provided to pump cooling air past the the oil cooler as well through the compartment. However, because of their multiplicity of high speed, rotating parts, these fans are susceptible to mechanical failures, which when they occur require that the aircraft be removed from operation. Further, these fans sometimes leak oil into the cooling flow which then covers the oil cooler fins resulting in reduced heat transfer and the possibility of an APU automatic shutdown because of excessive oil temperature. Also, as the oil cooler gets covered by this oil the flow of cooling air is blocked, backpressuring the fan and causing it to operate in a rotating stall which results in increased fan noise. Another disadvantage associated with fan cooler is that they increase the drag or load on the engine and therefore make starting the APU at the cold ambient temperatures encountered at high altitudes more difficult.
In addition to fans sometimes a simple eductor having a conic nozzle is added to the cooling system. This eductor utilizes the kinetic energy of the APU exhaust gas to entrain cooling flow through the compartment. Generally, these eductors are not capable of pumping sufficient air flow to cool an oil cooler, and are only used to provide tailpipe or compartment cooling. Further, because of the low air density at altitude, to generate sufficient cooling flow the area of the conic nozzle must be significantly closed down which produces a substantial backpressure of the APU and consequent loss of power. Another disadvantage with these eductors is that when surge bleed flow from the APU's compressor is dumped in the vicinity of the eductor, the eductor's effectiveness is reduced.
Accordingly, there is a need for a novel eductor that can entrain sufficient cooling or venting air flow through the compartment to provide all the necessary cooling including that required by an oil cooler and which can also receive surge bleed flow without reducing effectiveness.